Surge protection devices are conventionally designed for protecting circuitry from overvoltage surges. One type of conventional device provides temporary surge protection by shunting the overvoltage surge to ground or neutral. A prolonged overvoltage surge may cause such a surge protection device to overheat. An event of overheating can result in increased temperatures that can be above the combustion temperatures of nearby materials, causing burning of the materials. The excessive overvoltage can also result in arcing of the voltage across terminals in the circuitry, a condition which can rapidly result in unpredictable modes of damage.
A second type of surge protection device is connected in series between a voltage source and an active load. There is a problem that an overvoltage surge due to an excessively large voltage occurring across the terminals of the series-connected surge protection device may cause the surge protection device to suffer catastrophic failure. Flying debris from such a failure can facilitate arcing, resulting in subsequent damage of an unpredictable nature. For example, the arcing can cause melting of components and carbonization of the surfaces of the components, especially a printed wiring board surface. The carbonization can create additional current paths that can add to a destructive condition. Thus, it can be seen that catastrophic damage to an equipment installation, building, or occupants of a building can occur as a result of the excessive overvoltage.
One conventional method of reducing the risk of catastrophic damage is to enclose the components of a surge protector with a combustion-resistant material, such as dry electrical-grade silica. The dry electrical-grade silica displaces the air (which contains oxygen) within the cavity, reducing a risk of combustion, and also provides thermal capacitance to absorb the heat generated by the transient voltage spike, thus protecting adjacent equipment, even if the surge protection device is destroyed. However, prolonged heating and subsequent failure of the enclosure, even when it is filled with dry electrical-grade silica, causes a risk of fires. Although the ‘prolonged surge’ type of device may be improved to reduce a risk of heat and combustion, this conventional device using dry electrical-grade silica cannot prevent a catastrophic failure due to arcing caused by excessive voltage surges, especially not when the arcing occurs at input terminals of a power meter.
For a three-phase 480 volt service, an overvoltage surge can reach 10,000 to 20,000 volts or more. Such voltages cause different types of arcing depending upon the magnitude of the voltage, and depending upon variation in the properties of the several components in the surge protection circuit, including variation in adjacent materials and conditions. Thus, there is a problem in determining a location where arcing and catastrophic damage may occur.
More specifically, there is a problem that, for excessive overvoltage surges at the input terminals of an electric utility power meter, a mode of failure is unpredictable because arcing can occur due to a number of factors such as loose connections, wiring defects, stray current paths, and component failure, as well as being due to various conditions of the excessive voltage signal itself. The problem of unpredictability of a failure mode is compounded for an electric utility power meter, which is necessarily enclosed under the cover of the meter enclosure.
As discussed above, various types of surge protectors are well known. Surge protection devices generally have a nominal relatively high electrical impedance, which, upon being subjected to a voltage of sufficiently high magnitude, changes to a significantly lower impedance and conducts electrical current relatively readily. The various types of surge protection devices include varistors, silicon avalanche diodes, zener diodes, selenium cells, gas discharge tubes, and high voltage capacitors. Metal oxide varistors (MOVs) are used for many low voltage applications, with operating voltages of about six hundred volts AC (600 VAC) or less.
FIG. 1 illustrates a first conventional type of surge protection device, having a current limiting resistor 4, a metal oxide varistor (MOV) 5, and a polymeric positive temperature coefficient device (PPTC) 3, all disposed on a printed wiring board 1. The current limiting resistor 4 is coupled in series between a voltage source and the load 6 and the MOV 5 is coupled across the load 6 to ground. The MOV 5 acts to limit the voltage applied to the load 6. The current limiting resistor 4 serves to limit the maximum surge current through the MOV 5. The PPTC 3 limits the steady state current through the current limiting resistor 4 in order to prevent the resistor 4 from overheating. While the above device in FIG. 1 has some utility, it is limited the types of voltage sources to which it can be connected. Specifically, manufacturers of PPTC devices do not recommend a use of PPTC devices in a direct series connection with a utility power line as shown in FIG. 1. Moreover, excessively large currents such as those resulting from an overvoltage can cause destruction of the PPTC. As discussed above, such destruction can result in secondary arcing and unpredictable damage modes.
FIG. 2 illustrates a typical surge protection device used in a relatively high voltage device that is connected to a utility power line. In particular, the high voltage device is in an electrical utility meter. The conventional utility meter circuit connects the current limiting resistor 4 directly in series with the power line connection, voltage source 2. However, such a surge device can fail under certain conditions of excessive surge voltage, potentially resulting in secondary arcing as described further above. If a phase-to-phase or phase-to-neutral arc occurs, then the severity of the failure is unpredictable.
The conventional surge protection devices do not contain any protection for an arcing condition that may damage the protection device itself. There is a need for a surge protection device that inhibits catastrophic failure of the surge protector and otherwise provides a predictable failure mode.